During the last few years, when the universal aim of the designer and manufacturer has been directed to the highest efficiency of the product, a problem of a peculiar nature is sometimes met.
For a simple illustration, consider a pair of toothed wheels, or a chain and a sprocket, transmitting power. In either case, strength and durability are very essential in order to transmit the power safely and to retain the true surface of the tooth for the sake of good efficiency. It will be readily seen that, in order to meet such conditions satisfactorily, a material of a soft and tough nature should be employed - something that possesses strength and resistance to wear, and still conforms with standard practice of design regarding the proportions of parts. Such problems have come and have to be met by the manufacturer and the mechanic; they constitute the problem of casehardening. It is not a new subject but is one that is not well understood and not always easy to control. This statement is justified by the diversity of opinion among different investigators in regard to the fundamental principles underlying the process of casehardening.
The process consists, as the name implies, of forming a case of high carbon, which is capable of hardening a surface of a low-carbon steel or wrought iron which in itself has not the property of hardness. The prevailing theory is based on the following principles.
(1) The steel to be carburized must be placed in contact with carbonaceous material.
(2) The high temperature at which the steel and the carbonaceous material is heated gasifies the carbon (if in a solid state) and opens the pores of the steel, allowing the gaseous carbon to penetrate.
(3) The penetration is aided if the gas is under pressure and in the presence of nitrogenous matter acting as a carrier of the carbon.
The most important factor involved in casehardening is the carbonizing material) for upon this depends quality of case and uniformity of product. The great difficulty at the present time is to obtain a definite uniform result, and to duplicate it at will. Many investigators are engaged in the solution of this problem which eventually will be solved by the discovery of a reliable carbonizing material.
In the older process, charred organic matter - such as wood charcoal, charred leather, bone, horn, and the like - was used for packing. Charred leather being rich in nitrogen gave a very good result, but leather scrap became useful for other purposes, thus becoming unavailable for carbonizing. Granulated bone was next resorted to and is used extensively yet, but, owing to its high phosphorus content, it is apt to make parts brittle, and it should be avoided on small parts. Charcoal does not seem to find much favor, although it is fully as efficient as bone if properly prepared.
All of the materials mentioned are more or less unreliable for producing the desired effect on the steel. The question then arises, What would constitute a reliable material? Before replying directly, there must be considered the following factors which hinder the carburization of the steel by bone, charcoal, or similar prepared compounds.
(1) Unequal heating of the steel due to the manner of packing. It takes considerable time for heat to penetrate to the center containing the steel and packing; consequently, the parts near the walls of the crucible are heated sooner, and carbonizing begins earlier.
(2) Unequal composition of materials for packing causing variation in the amount of gas, and, therefore, unequal pressure generated in different sections of the crucible or packing box. To obviate this trouble, a packing material of perfectly uniform composition and high conductivity should be used. It should liberate carbon freely, yet not faster than the steel can absorb it. Steel absorbs carbon at certain rates depending on the temperature; high heat gives faster penetration. If an excess of carbon is liberated, the surface of the steel becomes supersaturated with carbon, the result being a brittle structure. The case should not be more than from 0.90 per cent to 1 per cent carbon, which is generally the result if proper heat and good carbonizing material are used. The packing material should be free from sulphur, especially if moisture is present, for sulphur acidifies moisture, and the combined effect produces a scaled pitted surface.
There are a number of casehardening compounds on the market claiming one or more of the ideal characteristics. In general these compounds consist of: carbon, volatile matter and hydrocarbon, nitrogen, ash, sulphur, and phosphorus. They are likely to contain silica, alumina, lime, ammonia, and carbonate.